The present invention relates to a power supply device for operating the back light of a liquid crystal display TV, and more particularly, to a device for supplying power to a small fluorescent light of a liquid crystal display TV such that after starting the back light, said light emits continuously without any noise generating therefrom and any variation of voltage source.
Heretofore, as shown in FIG. 1, in case of a push-pull voltage source converter type, when switching transistors TR.sub.1, TR.sub.2 are operated to ON/OFF, the switching voltage and current wave as shown in FIGS. 2(A) and 2(B) are transferred to load R.sub.L through transformer T.
At this moment, when said transformer T is an ideal transformer, switching operation of said transistors TR.sub.1, TR.sub.2 may be satisfied if controlled by a desired frequency from the exterior, however in this case, some switching control circuit means from the exterior is required additionally and the system becomes complicated, and therefore, it is disadvantageous and uneconomical for a device of small capacity such as liquid crystal display TV.
Further, in case of driving said switching transistors, an output voltage of alternating current can be obtained in case when switching operation is carried out by self oscillation, however, since the saturating property of the transformer is utilized, there is disadvantage that consuming electric power is larger.
That is, said switching operation is started after the power supply voltage V.sub.S is passed through resistor R.sub.S, and when the switching transister TR.sub.1 becomes conductive, voltage is applied to the transformer T and magnetic flux becomes increased and the transformer becomes saturated.
Therefore, primary current of the transformer T is rapidly increased, driving current is rapidly decreased and said transistor TR.sub.1 becomes OFF, consequently the voltage becomes inverted by magnetic current of said transformer T and making the transistor TR.sub.2 to be ON, and switching operation becomes repeated by operating principle of transformer as same as the operation of said transistor TR.sub.1, whereby output voltage and output current of square waves as shown in FIGS. 2(A), 2(B), 3(A), 3(B), 3(C) and 4 are produced, and therefore, there is disadvantage that consuming electric power is rapidly increased due to saturation of said transformer.
Said FIGS. 2(A) and 2(B) show the switching voltage and current wave forms in accordance with ON/OFF operation of transistors TR.sub.1, TR.sub.2, FIGS. 3(A) and 3(B) show switching voltage (FIG. 3(A)) and current (FIG. 3(B)) wave form in response to the load R.sub.L in the case when self-oscillating is occuring, FIG. 3(C) shows the respective cases of fully loaded and not loaded about a variable load, and FIG. 4 shows the output voltage and output current in response to the variation of the load, wherein the waveforms of V.sub.0, I.sub.L1 show the respective output voltage and current from the load in the case where the load has resistive and inductive components, and the waveform of I.sub.L2 shows the output current from the load in the case where the load has resistive and capacitive components.
FIG. 5 shows a conventional current source converter circuit, in which a power supply voltage V.sub.s is applied through coil L.sub.dc to the primary side of a transformer T, and when switching transistor TR.sub.1 is operated by a driving circuit, current is induced at the secondary side of said transformer T, according to said load R.sub.L, voltage and current applied over both end terminals of said transistor TR.sub.1 becomes to be as shown in FIGS. 6(A) and 6(B).
Further, switching the output voltage and current in response to the variation of said load are exhibited as voltage V.sub.01 and current I.sub.L from the load in the case where the load has resistive and inductive components as well as voltage V.sub.02 of the load in the case where the load has resistive and capacitive components as shown in FIG. 7.
However, since such current source converter utilizes also the characteristic that the transformer is saturated when executing operation, not only electric power loss at the transformer is large, but also particularly when switching, switching stress of the switching element is great and switching loss is also great, consequently high frequently raising and dropping are extremely limited and noise problem is much, and therefore, there is a problem that use is not suitable even for small liquid crystal display TV.